The present invention generally relates to wireless communication, and particularly relates to selecting signal processing delays from a candidate set of delays.
Multipath fading is a signal propagation phenomenon that results in a transmitted radio signal reaching at least one receiving antenna via two or more paths. Each received signal component experiences different attenuation, delay and phase shift while traveling from the source to the receiver. RAKE receivers attempt to counter the effects of multipath fading by placing several “fingers” at different signal path delays in order to receive individual components of a multipath signal. Each signal component is processed independently, but at a later stage combined in order to utilize the different transmission characteristics of each transmission path.
RAKE receivers are so named because of their analogous function to a garden rake, each finger being placed at a signal path delay for collecting bit or symbol energy similarly to how tines on a rake collect leaves. RAKE receivers tend to provide optimal interference suppression when signal interference and noise is white. However, multipath fading causes correlated interference.
Generalized RAKE (G-RAKE) receivers improve upon RAKE receivers in that they account for interference correlation by modifying finger placement accordingly. Unlike RAKE receivers which only place fingers on signal path delays for maximizing signal energy collection, G-RAKE receivers place some fingers on signal path delays and others off signal path delays. Fingers placed on signal path delays optimize signal energy collection while those placed off-path suppress interference.
One conventional approach for identifying an optimal group of delays in G-RAKE receivers is channel probing where a candidate set of delays is monitored and a subset selected for signal combining. Because wireless channels vary over time, the optimal delays used for signal combining also change. G-RAKE receivers use various conventional channel probing techniques to estimate the time-varying behavior of wireless channels. However, each of these techniques selects a subset of delays from a candidate set based on calculations instantaneously made at the channel fading rate.
For example, a candidate set of N delays may be used to probe a channel of interest. The M best delays are instantaneously selected at the channel fading rate based on a selection criterion such as maximal combining weights or signal-to-interference-plus-noise ratios. As such, delays are placed in optimal positions based on a present understanding of the wireless channel. The selected delays are then used to combine various components of a received signal, thus optimizing signal energy reception while suppressing interference. However, selecting delays at the same rate at which a channel changes increases the complexity of the receiver.